This invention relates to flash-spinning of polymeric, plexifilamentary, film-fibril strands. More particularly, this invention relates to flash-spinning of polymethylpentene.
U.S. Pat. No. 3,081,519 to Blades and White describes a flash-spinning process for producing plexifilamentary film-fibril strands from fiber-forming polymers. A solution of the polymer in a liquid, which is a non-solvent for the polymer at or below its normal boiling point, is extruded at a temperature above the normal boiling point of the liquid and at autogenous or higher pressure into a medium of lower temperature and substantially lower pressure. This flash-spinning causes the liquid to vaporize and thereby cool the exudate which forms a plexifilamentary film-fibril strand of the polymer. Preferred polymers typically include crystalline polyhydrocarbons such as polyethylene and polypropylene.
According to Blades and White, a suitable liquid for flash spinning (a) has a boiling point that is at least 25xc2x0 C. below the melting point of the polymer; (b) is substantially unreactive with the polymer at the extrusion temperature; (c) should be a solvent for the polymer under the pressure and temperature set forth in the patent (i.e., these extrusion temperatures and pressures are respectively in the ranges of 165 to 225xc2x0 C. and about 500 to 1500 psia (3447-10342 kPa); (d) should dissolve less than 1% of the polymer at or below its normal boiling point; and (e) should form a solution that will undergo rapid phase separation upon extrusion to form a polymer phase that contains insufficient solvent to plasticize the polymer.
Commercial spunbonded or flash-spun products have been made primarily from polyethylene plexifilamentary film-fibril strands and have typically been produced using trichlorofluoromethane as a spin agent; however, trichlorofluoromethane is an atmospheric ozone depletion chemical, and therefore, alternatives have been under investigation. There have been many other agents used for flash spinning polyethylene to either minimize or eliminate the potential for ozone depletion. Shin, in U.S. Pat. No. 5,032,326 discloses one alternative spin fluid, namely, methylene chloride and a co-spin agent halocarbon having a boiling point between xe2x88x9250xc2x0 C. and 0xc2x0 C. Kato et al. in U.S. Pat. No. 5,286,422 discloses an alternative, specifically, a spin fluid of bromochloromethane or 1,2-dichloroethylene and a co-spin agent of, e.g., carbon dioxide, dodecafluoropentane, etc.
As noted above, flashspun products have typically been made from polyethylene, however it is desirable to make flashspun products from other polymers, such as polymethylpentene that have the advantage of a higher melting point than polyethylene.
U.S. Pat. No. 5,250,237 to Shin mentions the use of alcohols with one to four carbons as spin agents for flash spinning polymethylpentene. Also, in a co-pending application assigned to DuPont (Docket No. TK-3315), certain azeotropic mixtures are used as spin agents for polymethylpentene. Regardless, a need exists to find additional solvents suited for polymethylpentene, yet also satisfy the need for non-flammability and zero or extremely low ozone depletion potential.
The present invention is a process for the preparation of plexifilamentary film-fibril strands of synthetic fiber-forming polyolefin which comprises flash-spinning at a pressure that is greater than the autogenous pressure of the spin fluid into a region of lower pressure, a spin fluid comprising (a) 5 to 30 wgt. % polymethylpentene, and (b) a spin agent selected from the group consisting of hydrochlorofluorocarbons; hydrocarbons; and chlorinated solvents.
This invention is also a spin fluid comprising (a) 5 to 30 wgt. % polymethylpentene and (b) a spin agent selected from the group consisting of hydrocarbons; hydrochlorofluorocarbons; and chlorinated solvents.
This invention is also directed to plexifilamentary film-fibril strands of fiber-forming polymethylpentene having a tenacity of at least 0.5 grams per denier and more preferably having a tenacity of at least 1 gram per denier. Also included are blends of polymethylpentene with polyethylene and polypropylene.
This invention is also directed to a process for the preparation of microcellular foam fibers from synthetic fiber-forming polyolefin which comprises flash-spinning at a pressure that is greater than the autogenous pressure of the spin fluid into a region of lower pressure, a spin fluid comprising (a) at least 40 wgt. % polymethylpentene and (b) a spin agent selected from the group consisting of hydrocarbons; hydrochlorofluorocarbons; and chlorinated solvents.
The invention is further directed to a process for the preparation of discrete plexifilamentary fibers (pulp) from synthetic fiber-forming polyolefins.